Image forming apparatus

Abstract

An image forming apparatus includes an image bearing member, a first belt to which a toner image is transferred, a first transfer roller in contact with an inner surface of the first belt, a second belt in contact with an outer surface of the first belt, and a second transfer roller to form a nip portion between itself and the first transfer roller through the second belt. First and second stretching roller are provided upstream and downstream of the second transfer roller in a rotational direction of the second belt, respectively. The first stretching roller includes a part, at both end portions in a rotational axis direction, of which a diameter is smaller than that of a central portion. The second stretching roller includes a part, at both end portions in the rotational axis direction, of which a diameter is larger than that of a central portion.

What is claimed is: 
     
       1. An image forming apparatus comprising:
 an image bearing member configured to bear a toner image; 
 a first belt to which the toner image is transferred from the image bearing member; 
 a first transfer roller provided in contact with an inner surface of the first belt and configured to form a nip portion where the toner image borne by the image bearing member is transferred to a recording material; 
 an upstream roller provided adjacently upstream of the first transfer roller with respect to the rotational direction of the first belt and configured to stretch the first belt while in contact with the inner surface of the first belt; 
 a second belt configured to form the nip portion while in contact with an outer surface of the first belt and to convey the recording material; 
 a second transfer roller configured to form the nip portion between the second transfer roller and the first transfer roller through the second belt and to transfer the toner image borne by the first belt to the recording material; 
 a first stretching roller provided adjacently downstream of the second transfer roller with respect to a rotational direction of the second belt and configured to stretch the second belt while in contact with an inner surface of the second belt; and 
 a second stretching roller provided adjacently upstream of the second transfer roller with respect to the rotational direction of the second belt and configured to stretch the second belt while in contact with the inner surface of the second belt, 
 wherein when a line perpendicular to a line connecting a center of the first transfer roller and a center of the second transfer roller and passing through a center of the nip portion is defined as a nip line L 1 , a tangential line of the first stretching roller passing through the center of the nip portion and formed along the second belt is defined as a tangential line L 3 , a tangential line of the second stretching roller passing through the center of the nip portion and formed along the second belt is defined as a tangential line L 2 , an angle formed by the nip line L 1  and the tangential line L 2  is defined as φ1, an angle formed by the nip line L 1  and the tangential line L 3  is defined as φ2, and an angle formed between the first belt, which is stretched between the first transfer roller and the upstream roller, and the tangential line L 2  is defined as φ3, 
 the first stretching roller, the second stretching roller, and the second transfer roller are disposed so as to satisfy φ1≤30°, φ2≤30°, and φ3<φ1, and 
 wherein the first stretching roller includes parts, at both end portions in a rotational axis direction, of which a diameter is less than a diameter of a central portion in the rotational axis direction in an area where the second belt is stretched, and the second stretching roller includes parts, at both end portions in the rotational axis direction, of which a diameter is greater than a diameter of a central portion in the rotational axis direction in an area where the second belt is stretched. 
 
     
     
       2. An image forming apparatus according to  claim 1 , wherein the first stretching roller has a crown shape of which the diameter decreases continuously from the central portion toward both end portions in the rotational axis direction, and
 wherein the second stretching roller has an inverted crown shape of which the diameter increases continuously from the central portion toward both end portions in the rotational axis direction. 
 
     
     
       3. An image forming apparatus according to  claim 1 , wherein a shape of the second stretching roller is set such that a peripheral length of which a part of the second belt is stretched in the central portion with respect to the rotational axis direction of the first stretching roller is substantially equal to a peripheral length of which a part of the second belt is stretched in the end portion with respect to the rotational axis direction of the first stretching roller. 
     
     
       4. An image forming apparatus according to  claim 1 , further comprising a third stretching roller configured to stretch the second belt while in contact with the inner surface of the second belt provided downstream of the first stretching roller and upstream of the second stretching roller with respect to the rotational direction of the second belt. 
     
     
       5. An image forming apparatus according to  claim 4 , wherein the third stretching roller is a tension roller for imparting tension to the second belt by an urging member being urged. 
     
     
       6. An image forming apparatus according to  claim 1 , wherein the second transfer roller is a driving roller for transmitting a driving force to rotate the second belt. 
     
     
       7. An image forming apparatus according to  claim 1 , wherein the first stretching roller and the second stretching roller are disposed such that a vertex of the first stretching roller is lower than a vertex of the second transfer roller and a vertex of the second stretching roller is higher than the vertex of the second transfer roller. 
     
     
       8. An image forming apparatus comprising:
 an image bearing member configured to bear a toner image; 
 a first belt to which the toner image is transferred from the image bearing member; 
 a first transfer roller provided in contact with an inner surface of the first belt and configured to form a nip portion where the toner image borne by the image bearing member is transferred to a recording material; 
 a second belt configured to form the nip portion while in contact with an outer surface of the first belt and to convey the recording material; 
 a second transfer roller configured to form the nip portion between the second transfer roller and the first transfer roller through the second belt and to transfer the toner image borne by the first belt to the recording material; 
 a first stretching roller provided adjacently downstream of the second transfer roller with respect to a rotational direction of the second belt and configured to stretch the second belt while in contact with an inner surface of the second belt, the first stretching roller including parts, at both end portions in a rotational axis direction, of which a diameter is less than a diameter of a central portion in the rotational axis direction in an area where the second belt is stretched; 
 a second stretching roller provided adjacently upstream of the second transfer roller with respect to the rotational direction of the second belt and configured to stretch the second belt while in contact with the inner surface of the second belt, the second stretching roller including parts, at both end portions in the rotational axis direction, of which a diameter is greater than a diameter of a central portion in the rotational axis direction in an area where the second belt is stretched; and 
 a third stretching roller configured to stretch the second belt while in contact with the inner surface of the second belt provided downstream of the first stretching roller and upstream of the second stretching roller with respect to the rotational direction of the second belt. 
 
     
     
       9. An image forming apparatus according to  claim 8 , wherein the first stretching roller has a crown shape of which the diameter decreases continuously from the central portion toward both end portions in the rotational axis direction, and
 wherein the second stretching roller has an inverted crown shape of which the diameter increases continuously from the central portion toward both end portions in the rotational axis direction. 
 
     
     
       10. An image forming apparatus according to  claim 8 , wherein the third stretching roller is a tension roller for imparting tension to the second belt by an urging member being urged. 
     
     
       11. An image forming apparatus according to  claim 8 , wherein the second transfer roller is a driving roller for transmitting a driving force to rotate the second belt. 
     
     
       12. An image forming apparatus according to  claim 8 , wherein the first stretching roller and the second stretching roller are disposed such that a vertex of the first stretching roller is lower than a vertex of the second transfer roller and a vertex of the second stretching roller is higher than the vertex of the second transfer roller.